halecomorphs

Halecomorphs are an extinct group of ray-finned fishes belonging to the order Halecomorphi. While this order is now represented by a single living species, the bowfin (Amia calva), it was once a diverse and widespread lineage during the Mesozoic Era. Halecomorphs first appeared in the Triassic period and reached their peak diversity during the Jurassic and Cretaceous periods. Fossil evidence suggests they inhabited both freshwater and brackish water environments across various continents.

The taxonomic classification of halecomorphs has undergone revision over time. Historically, they were sometimes grouped with teleosts, the largest and most diverse group of bony fishes, due to superficial similarities. However, more recent phylogenetic analyses, based on detailed anatomical studies of fossil specimens and molecular data from the living bowfin, have solidified their placement as a distinct lineage closely related to, but outside of, the true teleosts.

Key features that characterize halecomorphs include a long dorsal fin, a large gape, and ganoid scales, which are rhomboid in shape and interlocking, offering significant protection. Some extinct halecomorphs possessed adaptations for predatory lifestyles, while others may have been more generalized feeders. The decline of halecomorph diversity is generally attributed to the rise of the teleosts, which became increasingly dominant in aquatic ecosystems throughout the Cenozoic Era. The bowfin's survival is considered a testament to its successful adaptation to specific ecological niches.